1. Field of the Invention
This invention relates to processes for copolymerizing 2-pyrrolidone with caprolactam and to the resulting copolymer. In another aspect, this invention relates to processes for copolymerizing 2-pyrrolidone with caprolactam wherein said copolymerization is conducted in the presence of a crown ether and an N-acyl pyrrolidone.
2. The Prior Art
Poly-2-pyrrolidone (nylon-4) is composed of repeating C.sub.4 structural units ##STR1## This polymer can be molded into various useful shaped articles but is primarily of commercial interest because of its ability to be shaped or extruded into filaments suitable for synthetic fabrics. Similarly, polycaprolactam (nylon 6) can also be molded into various shaped articles, but is mainly used for synthetic fibers. The copolymers of 2-pyrrolidone with caprolactam generally correspond to poly-2-pyrrolidone with some of the C.sub.4 2-pyrrolidone structural units replaced with C.sub.6 caprolactam units and can also be made into filaments and other shaped articles.
Poly-2-pyrrolidone can be produced by the alkaline-catalyzed polymerization of 2-pyrrolidone in the presence of carbon dioxide, using a catalyst composed of alkali metal lactamate or tetraalkyl ammonium lactamate (see U.S. Pat. No. 3,721,652). Similarly, various alkaline-catalyzed copolymerizations of 2-pyrrolidone with caprolactam are described in J. Polymer Science Part A 1, 111 (1963) and Part A-1 5, 965 (1967), U.S. Pat. No. 3,017,393 (see Col. 4), and Czech Pat. No. 158,341. Copolymers of 2-pyrrolidone and caprolactam are also broadly named in U.S. Pat. Nos. 3,492,275 and 3,575,937 and lactam polymerization systems in which a mixture of 2-pyrrolidone and caprolactam is used as the solvent are described in U.S. Pat. Nos. 3,793,258 and 3,875,121. Chemical Abstracts, Vol. 59, 1801-B (1963) describes a publication by Volokhina et al disclosing the copolymerization of alphapiperidone with epsilon-caprolactam.
U.S. Pat. No. 4,073,778 is directed to an alkaline-catalyzed polymerization process for polymerizing lactams (e.g., 2-pyrrolidone) using crown ethers as initiators, and further generally discloses that additional initiators and/or activators, such as acetic anhydride or N-acyl lactamate may be used. This patent also broadly teaches that the process described therein may be applied to copolymerize different lactams, e.g., 2-pyrrolidone with caprolactam.
It has now been discovered that the presence of caprolactam in the reaction system described in U.S. Pat. No. 4,073,778 inhibits polymerization and greatly retards reaction rates and conversions. Accordingly, the present invention is directed to an improved copolymerization process which significantly increases reaction rates and yields and also substantially improves (increases) the molecular weight of the polymer product and the amount of caprolactam incorporated therein.
Crown ethers are macrocyclic polyethers which are neutral compounds containing 4-20 oxygen atoms each separated from the next by two or more carbon atoms. Macrocyclic polyethers have been found to form stable complexes with salts of alkali metals and other metals and ammonium salts: "Macrocyclic polyethers and their complexes", C. J. Pederson et al, ANGEW, CHEM. Internat. Edit., Vol. 11, page 16 (1972), and U.S. Pat. Nos. 3,562,295 and 3,687,978. The macrocyclic polyethers are believed to form salt-polyether complexes in which the cation is encircled by the oxygen atoms of the polyether ring and is held there by the electrostatic attraction between the cation and the negative ends of the CO dipoles. Since the stereo models of macrocyclic polyethers give a crown-like appearance, they are commonly designated as [N]-crown-M polyethers, wherein N is the total number of atoms in the polyether ring and M is the number of oxygen atoms in the polyether ring.
Crown polyethers ranging in size from cyclic tetramers of ethylene oxide ([12]-crown-4) and propylene oxide ([16]-crown-4) to 60-membered polyether rings (dibenzo[60]-crown-20) have been reported. The most effective complexing agents are said to be found among those polyethers containing 5-10 oxygen atoms each separated from the next by two carbon atoms.